The service instance is single-threaded and does not accept reentrant calls. If the InstanceContextMode property is Single, and additional messages arrive while the instance services a call, these messages must wait until the service is available or until the messages time out.

Reentrant

The service instance is single-threaded and accepts reentrant calls. The reentrant service accepts calls when you call another service; it is therefore your responsibility to leave your object state consistent before callouts and you must confirm that operation-local data is valid after callouts. Note that the service instance is unlocked only by calling another service over a WCF channel. In this case, the called service can reenter the first service via a callback. If the first service is not reentrant, the sequence of calls results in a deadlock. For details, see ConcurrencyMode.

Multiple

The service instance is multi-threaded. No synchronization guarantees are made. Because other threads can change your service object at any time, you must handle synchronization and state consistency at all times.

ConcurrencyMode is used in conjunction with the ConcurrencyMode property to specify whether a service class supports single-threaded or multi-threaded modes of operation. A single-threaded operation can be either reentrant or non-reentrant.

The following table shows when Windows Communication Foundation (WCF) permits an operation to be invoked while another one is in progress, depending upon the ConcurrencyMode.

The following code example demonstrates the different between using Single, Reentrant, and Multiple. This sample does not compile without a real implementation behind it, but does demonstrate the kind of threading guarantees that WCF makes and what that means for your operation code.

using System;
using System.ServiceModel;
[ServiceContract]
publicinterface IHttpFetcher
{
[OperationContract]
string GetWebPage(string address);
}
// These classes have the invariant that: // this.slow.GetWebPage(this.cachedAddress) == this.cachedWebPage. // When you read cached values you can assume they are valid. When // you write the cached values, you must guarantee that they are valid. // With ConcurrencyMode.Single, WCF does not call again into the object // so long as the method is running. After the operation returns the object // can be called again, so you must make sure state is consistent before // returning.
[ServiceBehavior(ConcurrencyMode = ConcurrencyMode.Single)]
class SingleCachingHttpFetcher : IHttpFetcher
{
string cachedWebPage;
string cachedAddress;
readonly IHttpFetcher slow;
publicstring GetWebPage(string address)
{
// <-- Can assume cache is valid. if (this.cachedAddress == address)
{
returnthis.cachedWebPage;
}
// <-- Cache is no longer valid because we are changing // one of the values. this.cachedAddress = address;
string webPage = slow.GetWebPage(address);
this.cachedWebPage = webPage;
// <-- Cache is valid again here. returnthis.cachedWebPage;
// <-- Must guarantee that the cache is valid because we are returning.
}
}
// With ConcurrencyMode.Reentrant, WCF makes sure that only one // thread runs in your code at a time. However, when you call out on a // channel, the operation can get called again on another thread. Therefore // you must confirm that state is consistent both before channel calls and // before you return.
[ServiceBehavior(ConcurrencyMode = ConcurrencyMode.Reentrant)]
class ReentrantCachingHttpFetcher : IHttpFetcher
{
string cachedWebPage;
string cachedAddress;
readonly SlowHttpFetcher slow;
public ReentrantCachingHttpFetcher()
{
this.slow = new SlowHttpFetcher();
}
publicstring GetWebPage(string address)
{
// <-- Can assume that cache is valid. if (this.cachedAddress == address)
{
returnthis.cachedWebPage;
}
// <-- Must guarantee that the cache is valid, because // the operation can be called again before we return.string webPage = slow.GetWebPage(address);
// <-- Can assume cache is valid. // <-- Cache is no longer valid because we are changing // one of the values. this.cachedAddress = address;
this.cachedWebPage = webPage;
// <-- Cache is valid again here. returnthis.cachedWebPage;
// <-- Must guarantee that cache is valid because we are returning.
}
}
// With ConcurrencyMode.Multiple, threads can call an operation at any time. // It is your responsibility to guard your state with locks. If // you always guarantee you leave state consistent when you leave // the lock, you can assume it is valid when you enter the lock.
[ServiceBehavior(ConcurrencyMode = ConcurrencyMode.Multiple)]
class MultipleCachingHttpFetcher : IHttpFetcher
{
string cachedWebPage;
string cachedAddress;
readonly SlowHttpFetcher slow;
readonlyobject ThisLock = newobject();
public MultipleCachingHttpFetcher()
{
this.slow = new SlowHttpFetcher();
}
publicstring GetWebPage(string address)
{
lock (this.ThisLock)
{
// <-- Can assume cache is valid. if (this.cachedAddress == address)
{
returnthis.cachedWebPage;
// <-- Must guarantee that cache is valid because // the operation returns and releases the lock.
}
// <-- Must guarantee that cache is valid here because // the operation releases the lock.
}
string webPage = slow.GetWebPage(address);
lock (this.ThisLock)
{
// <-- Can assume cache is valid. // <-- Cache is no longer valid because the operation // changes one of the values. this.cachedAddress = address;
this.cachedWebPage = webPage;
// <-- Cache is valid again here. // <-- Must guarantee that cache is valid because // the operation releases the lock.
}
return webPage;
}
}